This document discusses neutral grounding resistors (NGRs) and the importance of continuously monitoring them. It provides examples of problems that can occur if an NGR fails or loses connection, such as soft starter failures or transient overvoltages. Continuous monitoring can detect NGR issues and ensure ground fault protection still functions properly. An effective monitor measures resistance through the NGR to identify connectivity issues, and protects against failures that could leave systems unprotected from ground faults.

1. CMK
B.E(EEE)

2.  It is a grounding device
for non isolated system,
which is used to connect
neutral point of
transformer to earth
through resistance.
 Used to limit the fault
current in a generator or
a transformer during
earth faults.
Neutral grounding resistor.

3.  Reduces over voltages.
 Reduced equipment damage.
 Isolation of faulted circuit.
 Reduced operation/maintenance expense.
 Increase safety.
 Increased lightning protection.

4.  Failure mode is usually open circuit.
a) Desired fault current cannot flow.
b) Ungrounded system.
NGR

5. a) Undesired fault current can flow.
b) Solidly grounded system.
NGR
• NGR in short circuited mode.

6.  Properly designed resistance grounding can
avoid.
a) Eliminate transient over voltages.
b) Reduce the risk of an arc flash.
c) Provide continuity of service.
d) Provide accurate current for ground fault detection.

7.  Event discovery.
 NGR checking during
maintenance doesn’t
guarantee it’s working.
 Response to injected
current only ensures
the working of relay
only.
 NGR disconnected for
testing and do not
reconnect.
…..this ensures the need of CONTINUOUS NGR MONITORING
A primary current injection testing

8. (51N)- Over current measurement
(59N)- Over voltage measurement

10.  Possibility of
measuring continuity
through ground fault
 Resistance
measurement through
the faulted phase and
ground
 Continuity through a
fault
Continuity through a ground fault

11.  An over current
protection
 An over voltage
protection
 An resistance
measuring instrument

12.  Should detect NGR failure in spite of ground
fault
 Should work in tripping and alarm only
systems
 Monitoring neutral and ground connections
 Monitoring shouldn’t be
capacitively/inductively coupled to NGR
 Monitor should not be exposed to neutral
voltage during ground fault

15.  Soft starter failure
a) Soft starter operated in unexpected manner.
b) When the neutral properly grounded, all three
voltage signals are balanced.
c) Failure of the NGR allows the neutral voltage to float
and the three voltage signals to become unbalanced.
d) studying the situation the manufacturer concluded
that the problem was due to unbalanced reference
voltages.
e) further study and customer interviews it was
determined that an open NGR was responsible for
the unbalanced reference voltages.

16.  Loose connection of an NGR
a) The following incident occurred at a mine in
Eastern Canada on a 200 A, 4160V NGR.
b) NGR monitoring relay tripped on a resistor
fault.
c) Upon inspection, electricians noticed a loose
connection on the NGR
d) Eight other loose connections were found
and repaired.
e) Then resistor fault trip did not reoccur.

17.  The information presented here has shown that an open
NGR is an undesirable situation and current- sensing
ground-fault protection will not indicate the presence of a
ground fault. A well-designed NGR monitor provides
continuous protection against failures that previously
rendered ground fault protection, NGR , as well as leaving
the system exposed to damaging transient over voltages.
An NGR monitor provides confidence that current-sensing
ground-fault protection will operate as designed on the
next ground fault. A continuous NGR monitor is active
when control power is applied and indicates NGR health
whether or not the system is energized, with or without a
ground fault.

18. Thank you